Drug delivery systems are designed to provide a biocompatible reservoir of an active agent for the controlled release of the active agent dependent either on time, or on local conditions, such as pH. While macroscopic drug delivery systems such as transdermal patches, implantable osmotic pumps and implantable subcutaneous depots (e.g., NORPLANT™) have had some success, there has been continuing interest in microscopic drug delivery systems such as microcapsules, microparticles and liposomes.
Microcapsules and microspheres are usually powders consisting of spherical particles 2 millimeters or less in diameter, usually 500 microns or less in diameter. If the particles are less than 1 micron, they are often referred to as nanocapsules or nanospheres. A description of methods of making and using microspheres and microcapsules can be found, for example in U.S. Pat. No. 5,407,609. Microcapsules and microspheres can be distinguished from each other by whether the active agent is formed into a central core surrounded by an encapsulating structure, such as a polymeric membrane, or whether the active agent is dispersed throughout the particle; that is, the internal structure is a matrix of the agent and excipient, usually a polymeric excipient. The release of the active agent from a microcapsule is often regulated by the biodegradation of the matrix material, usually a biodegradable polymeric material such as either poly(DL-lactide) (DL-PL) or poly(DL-lactide-co-glycolide) (DL-PLG) as the polymeric excipient.
Liposomes can be considered microcapsules in which the active agent core is encompassed by a lipid membrane instead of a polymeric membrane. Liposomes are artificial lipid vesicles consisting of lipid layers, where the antigen may be encapsulated inside the aqueous compartment of the liposome, or associated with the antigen on the surface via surface-coupling techniques. Liposomes can be prepared easily and inexpensively on a large scale and under conditions that are mild to entrapped antigens. They do not induce immune responses to themselves, and are used in humans for parenterally administered drugs.
While the high surface area/volume ratio of microcapsules, microspheres and liposomes favor the release of the active agent, their small size provides challenges in manufacturing. A wide variety of methods to prepare microcapsules and microspheres are described in the literature, e.g., U.S. Pat. No. 5,407,609. Several of these methods make use of emulsions to make microspheres, in particular to make microspheres less than 2 millimeters in diameter. To give a general example of such processes, one can dissolve a polymer in a suitable organic solvent (the polymer solvent), dissolve or disperse an agent in this polymer solution, disperse the resulting polymer/agent mixture into an aqueous phase (the processing medium) to obtain an oil-in-water emulsion with oil microdroplets dispersed in the processing medium, and remove the solvent from the microdroplets to form microspheres. These processes can also be performed with water-in-oil emulsions and with double emulsions. The use of emulsion-based processes that follow this basic approach is described in several U.S. patents, such as U.S. Pat. Nos. 3,737,337, 3,891,570, 4,384,975, 4,389,330, and 4,652,441.
Alternatively, extracted yeast cell wall particles are readily available, biodegradable, substantially spherical particles about 2-4 μm in diameter. Preparation of extracted yeast cell wall particles is known in the art, and is described, for example in U.S. Pat. Nos. 4,992,540, 5,082,936, 5,028,703, 5,032,401, 5,322,841, 5,401,727, 5,504,079, 5,968,811, 6,444,448 B1, 6,476,003 B1, published U.S. applications 2003/0216346 A1, 2004/0014715 A1, and PCT published application WO 02/12348 A2. A form of extracted yeast cell wall particles, referred to as “whole glucan particles,” have been suggested as delivery vehicles, but have been limited either to release by simple diffusion of active ingredient from the particle or release of an agent chemically crosslinked to the whole glucan particle by biodegradation of the particle matrix. See U.S. Pat. Nos. 5,032,401 and 5,607,677.
Extracted yeast cell wall particles, primarily due to their beta-glucan content, are targeted to phagocytic cells, such as macrophages and cells of lymphoid tissue. The mucosal-associated lymphoid tissue (MALT) comprises all lymphoid cells in epithelia and in the lamina propria lying below the body's mucosal surfaces. The main sites of mucosal-associated lymphoid tissues are the gut-associated lymphoid tissues (GALT), and the bronchial-associated lymphoid tissues (BALT).
Another important component of the GI immune system is the M or microfold cell. M cells are a specific cell type in the intestinal epithelium over lymphoid follicles that endocytose a variety of protein and peptide antigens. Instead of digesting these proteins, M cells transport them into the underlying tissue, where they are taken up by local dendritic cells and macrophages.
M cells take up molecules and particles from the gut lumen by endocytosis or phagocytosis. This material is then transported through the interior of the cell in vesicles to the basal cell membrane, where it is released into the extracellular space. This process is known as transcytosis. At their basal surface, the cell membrane of M cells is extensively folded around underlying lymphocytes and antigen-presenting cells, which take up the transported material released from the M cells and process it for antigen presentation.
A study has shown that transcytosis of yeast particles (3.4+/−0.8 micron in diameter) by M cells of the Peyer's patches takes less than 1 hour (Beier, R., & Gebert, A., Kinetics of particle uptake in the domes of Peyer's patches, Am J Physiol. 1998 July; 275(1 Pt 1):G130-7). Without significant phagocytosis by intraepithelial macrophages, the yeast particles migrate down to and across the basal lamina within 2.5-4 hours, where they quickly get phagocytosed and transported out of the Peyer's patch domes. M cells found in human nasopharyngeal lymphoid tissue (tonsils and adenoids) have been shown to be involved in the sampling of viruses that cause respiratory infections. Studies of an in vitro M cells model have shown uptake of fluorescently labeled microspheres (Fluospheres, 0.2 μm) and chitosan microparticles (0.2 μm) van der Lubben I. M., et al., Transport of chitosan microparticles for mucosal vaccine delivery in a human intestinal M-cell model, J Drug Target, 2002 September; 10(6):449-56. A lectin, Ulex europaeus agglutinin 1 (UEA1, specific for alpha-L-fucose residues) has been used to target either polystyrene microspheres (0.5 nm) or polymerized liposomes to M cells (0.2 μm) (Clark, M. A., et al., Targeting polymerised liposome vaccine carriers to intestinal M cells, Vaccine. 2001 Oct. 12; 20(1-2):208-17). In vivo studies in mice have reported that poly-D,L-lactic acid (PDLLA) microspheres or gelatin microspheres (GM) can be efficiently taken up by macrophages and M cells. (Nakase, H., et al., Biodegradable microspheres targeting mucosal immune-regulating cells: new approach for treatment of inflammatory bowel disease, J. Gastroenterol. 2003 March; 38 Suppl 15:59-62).
However, it has been reported that uptake of synthetic particulate delivery vehicles including poly(DL-lactide-co-glycolide) microparticles and liposomes is highly variable, and is determined by the physical properties of both particles and M cells. Clark, M. A., et al., Exploiting M cells for drug and vaccine delivery, Adv Drug Deliv Rev. 2001 Aug. 23; 50(1-2):81-106. The same study reported that delivery may be enhanced by coating the particles or liposomes with reagents including appropriate lectins, microbial adhesins and immunoglobulins which selectively bind to M cell surfaces. See also, Florence, A. T., The oral absorption of micro- and nanoparticulates: neither exceptional nor unusual, Pharm Res. 1997 March; 14(3):259-66.
Pathogen pattern recognition receptors (PRRs) recognize common structural and molecular motifs present on microbial surfaces and contribute to induction of innate immune responses. Mannose receptors and beta-glucan receptors in part participate in the recognition of fungal pathogens. The mannose receptor (MR), a carbohydrate-binding receptor expressed on subsets of macrophages, is considered one such PRR. Macrophages have receptors for both mannose and mannose-6-phosphate that can bind to and internalize molecules displaying these sugars. The molecules are internalized by endocytosis into a pre-lysosomal endosome. This internalization has been used to enhance entry of oligonucleotides into macrophages using bovine serum albumin modified with mannose-6-phosphate and linked to an oligodeoxynucleotide by a disulfide bridge to a modified 3′ end; see Bonfils, E., et al., Nucl. Acids Res. 1992 20, 4621-4629. see E. Bonfils, C. Mendes, A. C. Roche, M. Monsigny and P. Midoux, Bioconj. Chem., 3, 277-284 (1992). Macrophages also express beta-glucan receptors, including CR3 (Ross, G. D., J. A. Cain, B. L. Myones, S. L. Newman, and P. J. Lachmann. 1987. Specificity of membrane complement receptor type three (CR3) for β-glucans. Complement Inflamm. 4:61), dectin-1. (Brown, G. D. and S. Gordon. 2001. Immune recognition. A new receptor for β-glucans. Nature 413:36), and lactosylceramide (Zimmerman J W, Lindermuth J, Fish P A, Palace G P, Stevenson T T, DeMong D E. A novel carbohydrate-glycosphinglipid interaction between a beta-(1-3)-glucan immunomodulator, PGG-glucan, and lactosylceramide of human leukocytes. J Biol. Chem. 1998 Aug. 21:273(34):22014-20). The beta-glucan receptor, CR3 is predominantly expressed on monocytes, neutrophils and NK cells, whereas dectin-1 is predominantly expressed on the surface of cells of the macrophages. Lactosylceramide is found at high levels in M cells. Microglia can also express a beta-glucan receptor (Muller, C. D., et al. Functional beta-glucan receptor expression by a microglial cell line, Res Immunol. 1994 May; 145(4):267-75).
There is evidence for additive effects on phagocytosis of binding to both mannose and beta-glucan receptors. Giaimis et al. reported observations suggesting that phagocytosis of unopsonized heat-killed yeast (S. cerevisiae) by murine macrophage-like cell lines as well as murine peritoneal resident macrophages is mediated by both mannose and beta-glucan receptors. To achieve maximal phagocytosis of unopsonized heat-killed yeast, coexpression of both mannose and beta-glucan receptors is required (Giaimis, J., et al., Both mannose and beta-glucan receptors are involved in phagocytosis of unopsonized, heat-killed Saccharomyces cerevisiae by murine macrophages, J Leukoc Biol. 1993 December; 54(6):564-71).